The invention relates to a process for measuring the heat output rate in a chemical reactor with the aid of a calorimeter, which comprises a measuring cell, an intermediate thermostat and a base thermostat, and taking as a basis the heat balance equation of the measuring cell as well as the heat balance equation of the intermediate thermostat.
The invention further relates to a calorimeter for effecting the process described above, comprising a measuring cell, which has at least one temperature detector, a controllable heating device with an output measuring circuit and an agitator, an intermediate thermostat, which has at least one temperature detector and a controllable heating device with an output measuring circuit, and a base thermostat, with the controllable heating device maintaining a constant temperature difference .DELTA.T.sub.1 between the base and the intermediate thermostat.
The technical field of the invention is, in its broadest sense, process instrumentation and control (process I & C). Control of large production reactors by modern methods is frequently effected by so-called model-assisted process management. Here, from a system of state variables (temperature, pressure, concentrations etc.), the unknown state variables are determined by determining the remaining variables and by applying mathematical methods, such as the Kalman filter or the Luenberger observer. The measured variable which may be made most readily available to said model-assisted measuring process in chemical processes is temperature. Other state variables, such as pressure, concentrations etc., are often only measurable with difficulty or their recording involves considerable time delays. A further measured variable which is analytically very meaningful is the rate of the change of enthalpy (heat output rate) of a chemical process, since each chemical elementary reaction is linked with a change of enthalpy of a greater or lesser magnitude.
The difficulties involved in measuring the heat output rate in a production reactor, i.e. a reactor which in the main may have a capacity ranging from around 100 liters up to several cubic meters, are considerable. Such a system is difficult to isolate from environmental influences. It is known that the temperature in such a production reactor is always fluctuating to some extent, that the input into the reaction mass of agitator output by any agitators and the heat losses are difficult to determine precisely.
Also, the effective heat capacity of the reactor content normally changes continuously in the course of reaction and the diathermancy of the reactor walls is similarly influenced by the addition of reaction mass, so-called "fouling".
Because of these difficulties, calorimetric variables are frequently measured using trial reactors having a capacity of between 0.1 and 15 liters, whose heat balance is easier to control. Translation of the variables thereby obtained to production reactors of the above-mentioned capacity is, however, always fraught with uncertainties owing to the changed volume-to-surface ratio and the impossibility of keeping to process management variables in a large-scale reactor. There is therefore a need to determine the state variables as far as possible in the production reactor itself, i.e. on line.
Such a process is described, for example, in Chem. Eng. Progress 81,9 (1985), page 57/61. This known process is based on the mass or heat balance equations of the actual production reactor and of its surrounding jacket through which coolant is circulated. Besides taking an incomplete heat balance equation as a basis, the known process also has the drawback that only a rough account is taken of any change in the effective heat capacity and in the density of the reactor content.